Diamond's superior properties, such as its hardness, high electrical resistivity, high thermal conductivity, optical transparency, low coefficient of friction and chemical inertness, make it advantageous for use in a wide variety of applications. A limitation on the use of diamond, however, has been the difficulty to bond diamond products to supporting structures, or to apply the diamond in the shape required by a particular application. Deposited diamond film has the potential to overcome these limitations. In electronics, possible uses of deposited diamond film include high power, high temperature semiconductors, or high heat conductivity PC boards for silicon integrated circuits. In cutting tools, diamond film can be used on drill bit cutters, bonded abrasives, or diamond grit, for example. Diamond film can be used to protect spectral or infrared windows in radomes. Diamond film can also be particularly useful with lead bond tools, or thermodes, which are used to apply heat to attach electrical leads onto semiconductor circuits.
The process of diamond film deposition is not completely understood. It is known, however, that diamond film can be grown at temperatures and pressures below the high pressure diamond forming region, typically between 40-80 kbars and 1,200.degree. C.-2,000.degree. C., from a vapor phase at low pressure, even though at equilibrium graphite is the most thermodynamically stable crystal form of carbon. To grow diamond film from a vapor under conditions where the graphitic crystal is preferred, it appears that carbon atoms on the substrate surface must be maintained in their sp.sup.3 bonding state. Carbon atoms bonded by sp.sup.3 bonds yield tetrahedral molecules which can form diamond crystals. Graphite is characterized by unsaturated sp.sup.2 bonding.
It has been found that the presence of atomic hydrogen suppresses graphite formation, probably by bonding with unsaturated carbon--carbon bonds. The reaction between hydrogen and the unsaturated carbon molecules breaks the sp or sp.sup.2 bonds.
Sufficient energy must be provided to produce reactive hydrogen atoms in the gas phase. Various methods have been used to energize mixtures of hydrogen and hydrocarbon gasses for diamond deposition. Diamond film has been grown on various materials by methods such as ion beam deposition, r.f. assisted chemical vapor deposition ("CVD"), and microwave assisted CVD. However, a reliable method for applying diamond film of adequate quality, at a sufficient rate, which can be used for the commercial production of a wide variety of diamond film products, has been elusive.
In U.S. Pat. No. 4,434,188, to Kamo, et al., a microwave is used to energize a vapor including hydrogen and a hydrocarbon gas and to heat the substrate to be coated. Kamo discloses a method for synthesizing diamond by plasma chemical vapor deposition wherein a microwave plasma of hydrogen gas is first generated, a substrate is placed into the plasma to be heated to between 300.degree. C.-1,300.degree. C. and a hydrocarbon is introduced into the plasma to be deposited and thermally decomposed on the substrate. Kamo states that the thermal energy decomposing the hydrocarbon must be sufficient to produce sp.sup.3 bonding between carbon atoms. A microwave oscillator generates an electrodeless discharge in an open tube chamber at reduced pressure to energize the plasma. The substrate is placed on a platform and positioned in the chamber so that it lies within the plasma. The substrate can be heated by the plasma, or a heater. A frequency of 300 MHz or above is used to generate the plasma.
In the "Diamond and Related Materials Consortium Newsletter" ("DRMC"), Vol. 1, No. 1, June 1987, on page 3, a similar open tube microwave plasma apparatus is described. An adjustable platform is included to better position the substrate. A moveable plunger or short circuit acts as a microwave reflector, increasing the power absorbed by the plasma and reducing the power reflected back to the oscillator. The system includes stub tuners for minimizing reflected power.
As discussed in the newsletter at pages 4-5, there are many drawbacks to such a system. For example, an asymmetrical plasma is formed at low pressure. While not stating the pressure level where this becomes a problem, an asymmetrical plasma results in non-uniform coating of a substrate. At high pressure, the plasma is confined to the center of the reactor, but the newsletter states that it would be desireable to control the size and shape of the plasma, independently of pressure. In addition, higher pressures decrease the size of the plasma, lessening the deposition area. With this system, only 2 square centimeters can be coated. Furthermore, the gas flows are set before the plasma is ignited. The substrate, therefore, is not at a sufficient temperature for diamond deposition at the start of a run. In Kamo, in contrast, the hydrocarbon gas is not introduced until the temperature of the substrate is above 300.degree. C. This temperature may be too low as well.
In addition, the substrate holders are etched by atomic hydrogen, introducing contaminants to the plasma and to the substrate. If the plasma is not confined to the center of the chamber, the walls of the tube are probably etched as well. Finally, many of the deposition parameters are linked. For example, the position of the plasma is controlled by varying the pressure.
A further apparent problem could be presented by the microwave generator, which emits a pulsed signal. During off cycles, the plasma is not being energized. This could interfere with the maintenance of the plasma, or allow the recombination of activated species.
Also described in the DRMC Newsletter, at pages 5-6, is a plasma assisted CVD system being developed by Applied Science and Technology, Inc. (ASTEX), which utilizes a bell jar to contain a plasma at low pressure within a resonant cylindrical waveguide. The system has limited tunability, providing some improvement over the open tube apparatus discussed above. A rectangular waveguide conveys microwave energy emitted by a continuous, 2.45 GHz generator to a cylindrical waveguide through a tuneable antenna. The rectangular waveguide can be tuned to change its mode and the antenna can be tuned to match impedances. The newsletter states that the applicator should be able to create a ball shaped plasma with a diameter up to 150 millimeters ("mm") and a height of about 200 mm, using a well defined plasma mode. It is stated that the position of the plasma can be controlled so that it does not come in contact with the walls of the bell jar, minimizing etching. Considering the size of the plasma, a relatively small surface is coated. This suggests that the system is inefficient. The applicator may be able to create a large plasma, however, the size of the plasma is not effectively used. While a large diameter allows larger surfaces to be coated, a large height has no apparent advantage. Such a large plasma is not necessary and wastes energy. This configuration also suggests that the microwave energy is not being effectively focused, implying that the tunability is limited.
In U.S. Pat. Nos. 4,507,588 ('558), 4,630,566, ('566), 4,585,668 ('668) to Asmussen et al., and U.S. Pat. No. 4,591,662, to Roppel et al., incorporated herein by reference, an improved ion generating apparatus and a method for treating surfaces is disclosed. The device includes provisions for tuning a microwave cavity by varying both its height and the depth of insertion of an adjustable excitation probe, to minimize reflected power and adjust the mode coupling. An electrically insulated plasma defining chamber, in the form of a quartz dish is used to contain the working gas at low pressure and produce a disk-like plasma within the microwave cavity. The working gas is injected into the chamber through a radial pipe. An ion attracting means adjacent the insulated chamber attracts ions from the plasma. A steel grid below the chamber provides a surface to reflect microwave energy. The properties of the plasma, such as its shape, position, density and electric field shape, are controlled by varying the input power, pressure, the height of the cavity and the position of the probe.
In the '566 patent, improvements are made to the apparatus to adapt it for use in etching, texturing, depositing or oxidizing a surface, such as an integrated circuit. For example, a platform for supporting an integrated circuit within the insulated chamber including the ion attracting means, is added. The diameter of the insulated chamber is also increased to accommodate a larger plasma for treating multiple surfaces. Also shown is a remote mode where ions are deposited outside of the insulated chamber. In addition, magnetic field coils can be placed around the cavity to produce a longitudinal electron cyclotron resonant magnetic field in the plasma. A multipolar magnetic field can be applied to reduce diffusion losses at low pressure. This can be particularly useful in remote mode deposition, outside of the insulating chamber.
The 566 patent also discloses a method for using the device to treat a surface including the steps of providing a tuneable ion generating apparatus which includes an electrically insulated chamber, forming a disk shared plasma in the chamber with a surface positioned to receive the ions in the plasma, and contacting the surface with the ions of the plasma. The method can additionally include attracting the ions to the surface with a biasing means.
The '668 patent states that a 10 centimeter ("cm") diameter plasma disk was maintained in a 17.8 cm diameter reactor at pressures between 10.sup.-4 torr to over 1 atmosphere, with inert gases, O.sub.2, H.sub.2 and gas mixtures. Input power varied between tens of watts to over 1,000, with less than 3% of the incident power being reflected. Silicon wafers were oxidized and the patent states that the method and device can also be used for thin film processing such as silicon deposition.